Variable band-pass filter circuits



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INVENTOR JAMES J LAMB w H Qs W May 2, 1939. J. J. LAMB VARIABL BAND-PASSFILTER CIRCUITS Filed July 9. 1957 M Y mxg FWI 1 M v v w 5. V Q $3552AAAAA vvvvvv ATTORNI-Y y 2, 1939- J. J. LAMB VARI ABLE BAND-PASS FILTERCIRCUITS Filed July 9, 1937 4 Sheets-Sheet 2 /I/IIINAN luw\\ (0 +5 '+'/0+/5 FROM RE80NANCE w 2 I "SEE figs l I I l I J l0 l5 Z0 25 30 .35 JAMESJ. LAMB TOTAL BANDW/DTH K C. BY 6 ATTORNEY--.

J. .1. LAMB 2,156,786

VARIABLE BAND-PASS FILTER CIRCUITS 4 Sheets-Sheet 3 May 2, 1939.

Filed July 9, 1957 70 #2 11 EAMI? I-Z DETEC'TOR \NVENTOR JAMES J. LAMBATTORNEY variable selectivity Patented May 2, 1939 UNITED STATES PATENTOFFICE 2,156,786 VARIABLE BAND-PASS FILTER CIRCUITS James J. Lamb, Westto Radio tion of Delaware Hartford, Conn, assignor Corporation ofAmerica,

a corpora- Application July 9, 1937, Serial No. 152,728

4 Claims.

This invention relates to variable band-pass filter circuits and hasmore particularly to do with a device having full-range superheterodyneselectivity in an intermediate frequency amplifier.

over 20 kilocycles. Furthermore, in order to cope with the wide varietyof interference conditions Accordingly, provide a band-pass filtersystem for use in an continuously 100 cycles to It is another object ofmy invention to provide In carrying out my invention I have found thatthe full-range encompassed may be covered in substantially three steps,each capable of giving continuously variable band-width between itsminimum and maximum limits. These are Such an assumption is quitereasonable as has my paper entitled Receiver selectivity characteristicspublished in Q. S. T., May 1935. i

For the highest selectivity range the familiar quartz crystal filter isused; for the medium'range I prefer to use what has been termed aTransfilter unit in place of the quartz crystal; and for thebroadest'range vari- The Transfilter unit has been article entitled Adescribed in my new I. F. coupling system for published in the April1937 This Transfilter unit comof a transformer and an Like the quartzfilter it is vbines the properties electric wave filter.electro-mechanical this bar has mechanically coupled to small plates ofRochelle salt.

Qualitatively the Transfilter displays the same electricalcharacteristics found in quartz resonance; that is, essentiallycapacitive reactance at most frequencies with a sudden deviation fromthis reactance at the frequency of resonance of Hence it may be used inbridge circuits in the same manner as a quartzfilten However, theRochelle salt driver steel bar has somewhat higher mechanical dampingwith the result that the filter 15 not as sharp as quartz; hence theusing such a filter in the intermediate band-Width of selectivity; TheTransfilter, on the other hand, provides much higher selectivity than isavailable with the use of audio tuned circuits.

lished in I. R. E., April 1922 and covered by United States Patent No.1,450,246.

The features of novelty of my invention are believed to be defined bythe appended claims. The invention itself, however, as to its structuraldetails and mode of operation, will be best understood upon reference tothe accompanying drawings when viewed in the light of the followingdescription. In the drawings:

Figure 1 shows a circuit diagram of a superheterodyne receiver embodyingan intermediate frequency amplifier and a combination of filter systemswhich may be selectively used for varying the band-width;

Fig. 2 shows a selectivity curve appropriate to the operation of thecircuit arrangements of Fig. 1;

Fig. 3 shows curves of total band-width obtainable by the operation ofthe circuit of Fig. 1;

Fig. 4 shows details of a modified arrangement of an impedance matchingTransfilter;

Fig. 5 shows for purposes of comparison a variation from the circuitarrangements of Fig. 4;

Fig. 6 shows a selectivity curve appropriate to the circuit arrangementof Fig. 4;

Fig. 7 shows total band-width curves for resistance variation;

Fig. 8 shows a modification of the invention in which a band-passcircuit is used comprising two Transfilters in parallel, and

Fig. 9 shows maximum and minimum selectivity curves in respect to thecircuit arrangement of Fig. 8.

Referring first to Fig. 1, I show therein an illustrative embodiment ofthe invention which includes a first detector stage tube I, a heterodyneoscillator 2, a first intermediate frequency amplifier 3, a secondintermediate frequency amplifier 4, a second detector 5, andinterconnecting circuit arrangements presently to be described in moredetail. The transformer 6 coupling the antenna circuit or radiofrequency amplifier to the first detector I may be of any conventionaltype. Transformers 1, 8 and 9 are preferably of the airtuned, air-coretype. The second detector 5 is preferably a double diode. The audiofrequency amplifier is connected to the output leads l0.

, The two intermediate frequency transformers 8 and 9 are adjusted for arelatively broad frequency characteristic to provide a fair amount oftolerance near resonace thereby to accommodate minor deviations infrequency of the several piezo-electric or Transfilter devices used.

As shown in the drawing, the first intermediate frequency stage ispreceded by an arrangement of alternative filter circuits. For theband-width of sharpest selectivity the piezo-electric crystal H isswitched into circuit between one terminal of the secondary of thetransformer I and the adjustable coupling condenser l2. The switchingarrangement includes a double-pole, triple-throw switch indicatedgenerally at l3. The crystal II is connected in circuit with the switcharms if in the position shown. In the intermediate position of theswitch arms a Transfilter unit I4 is substituted for the quartz crystal.The steel bar l5 of the Transfilter unit is grounded. In the thirdposition of the switch l3 both the quartz crystal I I and theTransfilter unit are cut out and tuning is obtained for the broadestselectivity band by merely short-circuiting the switch points Ill.

The variable capacitor l1 in shunt with the secondary of the transformerI is used to control the band-width or selectivity. Another variablecapacitor 18 in opposition to the filter units controls the rejection ofinterfering signals. In varying the band-width by adjustment of theparallel tuned impedance as indicated in the diagram, maximum band-width(minimum selectivity) occurs with this circuit tuned to crystalresonance and decreasing band-width (increasing selectivity) occurs asthe parallel tuned circuit becomes reactive on either side of resonance.With the impedance matching which this circuit provides, the over-allcarrier wave gain of the receiver is practically the same with the inputcircuit adjusted for optimum (medium high) selectivity as it is with thecrystal shorted out and the input circuit adjusted for maximum straightsuperhet gain. Either side of this point the over-all gain decreasesslightly, both toward maximum band-width and toward extreme minimumband-width.

The Transfilter unit I4 is of fairly low impedance and accordingly cutsthe gain of the input amplifier or first detector when fed directly fromits anode Hi. When the Transfilter is used by setting the switch |3 toits intermediate position, selectivity is varied by the same method aswhen the crystal filter is used; that is, by variation of the paralleltuned impedance which constitutes the input to the divided circuit.Although the selectivity control condenser l1 settings are not exactlythe same as for a quartz crystal filter of corresponding frequency,minimum selectivity occurs with the input circuit resonant to theTransfilter frequency and increasing selectivity occurs as the inputcircuit is tuned either side of resonance. The resonance setting(maximum band-width) comes at lower tuning capacitance with theTransfilter than with the crystal because the Transfilter capacitance toground is apparently greater by as much as 10 micro-micro-farads. Theadjustment is still well within the range of the condenser, however.

Many of the details of the circuit arrangements shown in Fig. 1 areself-explanatory but in some respects they follow the teachings of myPatent No. 2,054,757, granted September 15, 1936, reference to which maybe made for further explanation of these details.

The range of selectivity obtainable with the crystal filter circuit isshown by curve A in Fig. 2 as a maximum and by curve B when the crystalis adjusted for minimum selectivity. Between these two curves A and B acharacteristic shading has been introduced for representing theband-width variation under different adjustments. A different shadinghas been used to show the band-width variation under the differentadjustments between a maximum selectivity limit 0 and a minimumselectivity limit D when the Transfilter is used. Curve E is for thetransformer-coupled selectivity characteristic of the intermediatefrequency amplifier without either filter, that is, when the switch I3is moved to the extreme right for short-circuiting the contact pointsHi.

It is interesting to note that the selectivity range with theTransfilter practically continues on from where the crystal rangereaches its broadest. This is illustrated even more clearly by the totalband-width curves of Fig. 3 which are plotted from the same experimentaldata as used in plotting the curves of Fig. 2. The principal differencebetween the selectivity of the crystal filter at its broadest and theTransfilter at its sharpest is that the Transfilter selectivitycharacteristic is somewhat broader near reso- Transfilter selectivitynance. width. Under practical working conditions the circuitarrangements of my invention as herein dehave been shown to be wellgiving a slightly greater effective bandsponse characteristic forelimination of a particular interfering carrier evenwithin the normalband-width range.

range carries on from this point to a band-width sufiibiently great forspeech reception with entirely adequate fidelity. In

fact, the Transfilter selectivity at its broadest is generally usefulfor broadcast program reception,

providing fidelity fully as good as those customary with the averagebroadcast receiver.

This rangeis especially adapted to short-wave broadcast reception whereit is desirable to conference which is aggravated by the fading socharacteristic of these frequencies. True highfidelity reception ispractically never feasible on the high-frequency bands, and considerablehighquency band-width control with the Transfilter in much moresatisfactory fashion than it can be obtained by an audio-frequency tonecontrol accomplishes the same eiTect of reducing the noise but does sowithout introducing the amplitude distortion which may occur withaudio-frequency tone control. Furthermore, it does the job prior to thesecond detector and removes noise and adjacent-channel sidebandcomponents before they have a chance to intermodulate with the desiredsignal in the second detector to produce low-frequency audio componentswhich cannot be removed byaudio-frequency filtering subsequent todetection.

A matter of some importance in judging the relative merits of selectiveintermediate frequency circuits, in addition to their contribution ofselectivity, is their effect on the over-all gain and effectivesensitivity. In connection with crystal filters, for instance, there isconsiderable divergence of opinion as to whether this or that particulararrangement is the better in point of how little it reduces the gain ofthe receiver. In my experience, the impedance-matching crystal tion.This refers particularly to the carrier wave v by adjustment provided,

On the other hand, the

output is less, as is also the effective sensitivity of the receiver. I

When using the Transfilter, the gain is also negligibly afiected .ascompared to the straight superhet gain. In practice, differences of afew decibels in over-all gain are readily compensated of the receiversgain control of course, the receiver has a proper margin of surplusamplification to start with. This should be true with any good receiverhaving a two-stage intermediate amplifier;

Of more importance than gain is the effective sensitivity of thereceiver. This effective sensitivity is by no means. a simple matter ofhow much amplification the receiver has. It is, rather, a matter ofsignal-noise ratio. It is best expressed in terms of the receivers noiseequivalent. The noise equivalent is the signal input required to givesignal power output equal to the noise power output. The noise concernedis the receiver "hiss noise, which would be the lowest possible noisebackground under ideal receiving conditions. The noise equivalent willbe determined primarily by the signal-noise ratio at the input of thereceiver but will be afiected by the subsequent selectivity because thenoise power output is generally reduced in proportion to the reductionin effective bandwidth of the receiver.

In tests which have been made with the filter system of my invention ithas been observed that a considerable improvement was obtained uponincreasing the selectivity. In the case of carrier wave reception withthe crystal filter at maximum selectivity, for instance, the sensitivityis about 700 percent of the straight superheterodyne sensitivity, whilethe speech and music sensitivity with the Transfilter'set for sharp bandselectivity is raised to over 300 percent. The broad band selectivityadjustment of the crystal filter is likewise raised to over 300 percent.

In the range of adjustment of the selectivity or band-width control withthese circuits, the resonance frequency of the crystal filter varies buta few cycles. This variation is so small that if the .Ignal is first.tuned in with the crystal set selectivity.

Referring now to Fig. 4, I show an arrangement whereby the selectivitymay be adjusted by varying the resistance in the ground connection forthe Transi'ilter. This arrangement including a potentiometer it providesan impedance matching circuit. When the resistance is entirely out outthe circuit 1. In this position maximum selectivity is obtained if thecondenser C1 is adjusted for slightly higher capacitance than theresonance setting.

The impedance adjustment between the Transfilter and ground as shown in5 is found to be suitable when the transformer secondary has a lowimpedance instead of the divided capacitance step-down arrangement usedin Fig. 4. The circuit arrangement of Fig. 5 is, however, less flexiblethanthat of Fig. 4 and, in fact, when used with a crystal filter inplace of the Transfilter the circuit becomes one of the fixedselectivity type.

Fig. 6 shows a selectivity curve, the datafor which was plotted fromtests made with the circuit arrangement shown in Fig. 4. .The curves forzero resistance and for 2500 ohm resistance are not shown since theypractically coincide with the 1000 ohm curve. The most interestingfeature of these selectivity curves is the notch which appears with20,000 ohm resistance. This double-hump effect indicates the equivalentof over-coupling with a transformer. As compared to the selectivitycurves of Fig. 2, it is apparent that increased resistance tends tobroaden the nose of the selectivity characteristic less effectively,while the skirts of the curves spread out more rapidly. They also showthat the selectivity characteristic is generally less symmetrical withresistance variation than with variable impedance control. The curves ofFig. '7 show the total bandwidths for the various values of theresistance.

The gain of the circuit falls off somewhat more rapidly with increasingbandwidth as compared to the gain variation with impedance control ofselectivity, although the loss is not especially noticeable in practice.On the whole, adjustable impedance control of selectivity appears to bepreferable to resistance control with the Transfilter, just as it hasbeen found to be preferable with the quartz crystal filter.

An interesting band-pass type of selectivity characteristic was obtainedwith two similar Transfilter units connected in parallel in the circuitof Fig. 8. Except for the additional unit, the circuit is identical withthat of Fig. 1. The two units had the same rated frequency of 465kilocycles and actually differed only 200 cycles in resonance frequency.The band-pass curve of A of Fig. 9 was'obtained with the band-widthcontrol condenser C1 critically adjusted so that the same output wasobtained on both humps with constant signal input. The mid-frequency ofthis selectivity curve is approximately 1.2 kilocycles lower than themaximum-selectivity curve obtained with the input condenser C1 ad-.

justed for slightly greater capacitance than the broad-band adjustment.The greater broadening of the selectivity curve near resonance isespecially desirable in broadcast program reception, although theover-all carrier wave gan with this circuit is practically the same aswith a single unit.

Among other variations which I have tried, a particularly interestingone is use of a variableselectivity Transfilter and a quartz crystalfilter of the same type in cascade; that is, the crystal filter circuitas the coupling element between the first detector and the firstintermediate frequency amplifier, and the Transfilter as the couplingelement between the first and second intermediate frequency amplifiers,provision being made to switch either one in and out. A notableimprovement with the Transfilter adjusted for medium selectivity is thatthe crystal filter selectivity characteristics are steepened in theskirts. While such cascade filters require fairly close tolerances inthe resonance frequencies of the Transfilter and crystal, there appearsto be no great difficulty in meeting the requirements with productiontypes. The fact that the Transfilter frequency can be shifted over arange of a few hundred cycles, by tuning the input circuit above orbelow resonance, aids in accomplishing close alignment. Tests on threesample production-type Transfilter units have shown a maximumresonancefrequency difference of 380 cycles, the variation being a plusor minus 200 cycles or less from the average.

Further interesting and useful selectivity characteristics are obtainedwith two variable-selectivity crystal filter circuits similarly incascade. With one filter adjusted for minimum selectivity and the otherfor optimum selectivity, for instance, independent rejection control incarrier wave reception makes it possible to eliminate two interferingheterodynes of different frequencies, whether both are on the same sideof resonance or on opposite sides of resonance. The crystals may differ100 cycles or so in frequency without appreciably impairing operation,it has been found. In fact, such a difference actually may proveadvantageous, since it gives a band-pass characteristic in the regionnear resonance.

Fig. 9 shows maximum and minimum selectivity curves obtained with theband-pass filter circuit of Fig. 8. The mid-frequency of curve A isapproximately 1.2 kilocycles lower than the resonance frequency of curveB. Curve C is the straight superheterodyne selectivity curve without thefilter. The selectivity curves are otherwise self-explanatory.

In carrying out my invention it will be apparent that variousmodifications may be made by those skilled in the art without departingfrom the spirit and scope of the invention itself. It is to beparticularly understood that the curve diagrams are in themselves merelyexemplary of certain circuit arrangements and that where the values ofdifferent components in these circuits are varied, different curvediagrams would be produced. The practical application of the inventionwill be particularly appreciated where systems of the type herein shownare adopted for short-wave reception and other amateur use. The scope ofthe invention is, however, by no means limited in that manner.

I claim:

1. An electric wave filter system comprising a plurality of selectivelyoperable band-pass circuits one of said circuits including a quartzpiezo-electric device, another of said circuits including a mechanicallyresonant element in association with input and output piezo-electricelements of Rochelle salt, means including a variable impedanceeffectively in series with a selected one of said circuits for varyingthe frequency band width of said filter system, and means in shunt withsaid selected one of said circuits for applying thereto a phasedisplaced voltage whereby undesired signal frequencies are suppressed.

2. An intermediate frequency amplifier stage in combination with afilter system as defined in claim 1 and including switching means forselectively inserting a particular one of said bandpass circuits inseries with said amplifier stage.

3. In an electric wave filter for a superheterodyne receiver, the methodof varying the bandpass width and, conversely, the selectivity, whichcomprises feeding the energy to be filtered through one or another ofseveral paths having different selectivity characteristics, thecharacteristic of one of said paths being due primarily topiezo-electric action, and the characteristic of another of said pathsbeing due to a combination of mechanical resonance and piezo-electricaction, subjecting said energy to a series impedance, varying saidimpedance between a predominantly inductive and a predominantlycapacitive value, thereby to tune the same on either side of theresonant frequency of said energy and Width being filtered.

4. An electric wave filter comprising an input circuit, an outputcircuit, circuit means comprising a selective electro-mechanicaltransducer having piezo-electric input and output coupling elements fortransferring energy from said input to said output circuits, 2, variableimpedance undesired signal frequencies are suppressed.

JAMES J. LAMB.

